The present invention relates generally to a Traffic Alert and Collision Avoidance System (TCAS) and, more particularly, to an improved TCAS wherein target position is augmented by radar. In a further aspect of the invention, TCAS protection is provided to aircraft employing the present invention against all aircraft, independently of the type of transponsder a target aircraft is carrying, including target aircraft that do not have an operating transponder.
TCAS is a family of airborne devices that function independently of the ground based air traffic control system, and provide collision avoidance protection for a broad spectrum of aircraft types. The function of the TCAS is to present air traffic information, declare threats, and provide advisories on potential air traffic conflicts before the closest point of approach.
TCAS I provides proximity warning only, to assist the pilot in the visual acquisition of intruder aircraft. TCAS II provides traffic advisories as well as recommends escape maneuvers, or resolution advisories, for avoiding conflicting traffic. Resolution advisories are provided in the vertical direction only. Future enhancements to TCAS, which are still under development, are intended to provide traffic advisories and resolution advisories in both the horizontal and vertical directions.
The TCAS typically uses a Mode S transponder with which aircraft may be equipped to communicate collision avoidance decisions between aircraft. TCAS II, which is intended for large commercial and general aviation aircraft, tracks aircraft in proximity in both the horizontal and vertical planes, from which the time of closest approach may be predicted. The pilot of the interrogating aircraft is advised on how to avoid collision with a vertical resolution advisory, to climb, descend, do not climb, do not descend, and may also be advised as to a required vertical speed to avoid a collision. If the threat or intruder aircraft is also equipped with TCAS, the avoidance maneuver will be communicated and coordinated with the threat aircraft accordingly, so that both aircraft do not select the same avoidance maneuver.
Threat detection is performed at two levels. When air traffic approaches within a predetermined protective volume, a traffic advisory is provided to alert the pilot to attempt a visual siting and take whatever corrective action he or she deems appropriate to increase separation between the aircraft. If the two aircraft continue to approach, typically such that within a minimum of 20 seconds they will be within 0.1 nautical mile of each other with an altitude difference of not greater than 750 feet, a resolution advisory will be issued, displaying a vertical maneuver to the pilot to increase separation of the aircraft. The advisories may be accompanied by limits to the rate of climb or descent. TCAS equipment typically provide a graphical representation of the positions of the two aircraft and a numerical indicia of vertical separation. The graphical resolution advisory may be accompanied by an aural advisory, such as "descend, descend," etc.
One drawback of present systems is that the lack of bearing measurement accuracy precludes support of horizontal maneuvers. Another drawback of present systems is that the level of protection provided by TCAS depends on the type of transponder the target aircraft is carrying. TCAS provides no protection against aircraft that do not have an operating transponder.
Aircraft separation is central to the concept of "Free Flight." Improved technologies for ensuring separation are imperative for the transition of the National Airspace System from the current centralized command-and-control system between pilots and air traffic controllers to free flight, a distributed system that allows pilots, whenever practical, to choose their own route and file a flight plan that follows the most efficient and economical route.
It would, therefore, be desirable to provide an improved TCAS which is capable of providing sufficiently high accuracy to provide resolution advisories in both the vertical and horizontal directions for avoiding intruding aircraft. It would also be desirable to provide collision avoidance protection against all aircraft, even those that do not have an operating transponder.